In conventional coating methods for coating a running flexible support (hereinafter referred to as "a web") with a coating composition such as a magnetic coating composition or the like, for example, an extrusion-type coating apparatus, a curtain-flow-type coating apparatus, a blade-doctor-type coating apparatus, a slide-coat-type coating apparatus, etc., are generally used. Of those methods, a coating method using an extrusion-type coating apparatus which can realize coating of an uniform thin layer has been used in various fields.
According to such an extrusion-type coating apparatus, however, it is impossible to obtain a wide range of good coating conditions so that there has been a problem in that desired coating cannot be performed under the conditions of high viscosity, thin film coating, and high speed coating. Particularly, in a method of producing a magnetic recording medium. The following conditions exist. The thickness of a web is extremely thin. The ferromagnetic coating composition has a so-called thixotropy. Also, a magnetic substances are improved as a result of advancement in making a magnetic recording medium have a high recording density so that oxidized magnetic powder of a high S.sub.BET or a barium ferrite material is used so as to make a magnetic coating composition more viscous. Accordingly, there is a tendency that longitudinal stripes of color unevenness (thickness unevenness) occur more frequently as the coated layer becomes thinner and coating speed becomes higher.
This is because, if the inner diameter of a reservoir of a coating head is too small, or the coating width of a slit is too wide, the pressure of the coating composition in the reservoir is reduced largely from the coating composition supply side toward the opposite side, so that the quantity of the coating composition ejected from the slit at the coating head top end portion changes largely in the direction of the width of the coating head.
As a result of analyzing these problems, there has been a proposal as disclosed in Japanese Unexamined Patent Publication No. Hei-1-180266. According to this proposal, in an extrusion-type coating head, the quantity of coating, the gap quantity of a slit, the length of a slit, the ejection width of the slit and the inner diameter of a reservoir are established so as to satisfy a predetermined relational expression to thereby restrain the above-mentioned change of coating quantity in the width direction of a coating head to make the film thickness of a coating film uniform.
If a coating apparatus is established according to such a relational expression, however, it is necessary to make the slit length extremely long, or to set the slit gap to be extremely narrow. That is, the nozzle portion of the coating apparatus becomes very large in the former case, while in the latter case foreign matter tends to be trapped in the slit so that a great number of longitudinal stripes are generated in the coated surface. Thus, either of the former and latter cases is not practical. Further, although the above-mentioned relational expression is established on the assumption that the coating composition obeys poiseuille flow in the reservoir, any practical coating composition is so strongly non-Newtonian that the apparent viscosity dominating its liquid property is changed by the flow speed, and the apparent viscosity is also changed in the reservoir or in the slit, so that it is still difficult to obtain uniform film thickness even if the coating apparatus is constructed according to the above-mentioned relational expression using parameters of only the sizes of respective parts of the coating apparatus.
Therefore, a further proposal was disclosed in Japanese Unexamined Patent Publication No. Hei-3-202171 by it is possible to provide a coating apparatus in which the coating quantity, the slit gap quantity, the slit length, the slit ejection width, the inner diameter of the reservoir, and the viscosity of the coating composition are established so as to satisfy a predetermined relational expression, and the liquid property due to thixotropy at the time of performing coating with a coating composition is taken into consideration so that generation of color unevenness or longitudinal stripes is effectively restrained, a coated layer having superior quality to that of conventional layers can be formed, and magnetic recording media excellent in electromagnetic conversion property can be manufactured.
However, even in such a coating apparatus, to obtain a uniform film thickness it is necessary to adhere strictly to the predetermined relational expression. The shape of the coating head thus greatly depends on the specific coating conditions such as the coating film thickness, the coating speed, the kind of the coating composition, and so on. That is, even if a coating apparatus is provided according to the relational expression disclosed in Japanese Unexamined Patent Publication No. Hei-3-202171, it is necessary to reconstruct the coating head when the above-mentioned coating conditions are changed, thus resulting in a problem in that the cost to produce the coating head is increased. Each coating head is severely limited to use only under certain specific coating conditions.